Gelable liquid and method for selectively inhibiting the gelation of a gelable liquid

ABSTRACT

A controlled-gelation aqueous solution is proposed. The solution contains a hydrophobically-aggregating gelling agent, and an effective amount of an inhibitor which suppresses hydrophobic aggregation of the gelling agent. The inhibitor is soluble in water so that when the controlled-gelation aqueous solution contacts aqueous media the inhibitor disperses and gelation occurs. However, the inhibitor is substantially insoluble in hydrocarbons so that when the controlled-gelation aqueous solution contacts hydrocarbon media the inhibitor continues to suppress said hydrophobic aggregation and gelation is suppressed. The solution may be used to control the flow of water into oil wells, making it possible to reduce the water cut.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Ser. No. 10/340,444, filed Jan.10, 2003, which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates to a gelable liquid which is capable ofselective gelation depending on the type of medium (aqueous orhydrocarbon) with which it is in contact. The present invention alsorelates to a method for selectively inhibiting the gelation of a gelableliquid.

BACKGROUND OF THE INVENTION

In most oil wells, water enters the well and is recovered together withthe oil. Furthermore, as the well ages the amount or cut of recoveredwater generally increases. On average, about 3 tons of water per one tonof oil are pumped out of depleting oil formations [1]. The size of thewater cut produced by the well has a substantial effect on the economicsof well operation.

Thus methods for limiting water inflow have been developed. These makeit possible to reduce substantially the cost of oil recovery and toincrease the amount of oil extracted from the formation.

One method of limiting the inflow of water involves the injection intothe well of liquids capable of selectively reducing the flow of waterwithout impeding the flow of oil [2]. Ideally, during injection into thewell the liquid has a relatively low viscosity. Subsequently the liquidshould form a gel “plug” in those zones of the well from which there isan inflow of water, but gelation should not occur upon contact of theliquid with oil.

The active component of gelable liquids can be provided byhydrophobically associating substances which are capable of formingphysical gels in aqueous media [2, 3]. Such substances may be e.g.hydrophobically modified polymers, viscoelastic surfactants andpolymer/surfactant complexes [2, 3].

The formation of physical gels from hydrophobically modified polymers isdescribed in papers [4-6]. The formation of physical gels fromviscoelastic surfactants is described in papers [7-9]. Gelation inpolymer/surfactant complexes is described in papers [10-12].

A problem associated with gelable liquids intended to have selectiveplacement capability and based on hydrophobically associating substancesis that these substances tend to form physical gels immediately afterbeing introduced to water. It is then difficult to pump the liquid intothe well because formation of a physical gel has already commenced,increasing the viscosity of the system. Another problem is that contactbetween the physical gel and hydrocarbons may not result in destructionof the gel, leading to plugging of oil-bearing formations. The latterproblem is particularly associated with gels formed by hydrophobicallymodified polymers. Thus it would be desirable to be able to: (1)sufficiently slow down the process of gelation in aqueous media suchthat the gelable liquid has a low viscosity during injection, (2)nonetheless form a gel downhole on contact between the liquid and water,and (3) suppress gelation on contact between the liquid and oil.

Several methods for slowing gelation have been proposed for gelableliquids comprising viscoelastic surfactants [3]. However, as far as thepresent inventors are aware, methods for controlling the speed offormation of physical gels in gelable liquids comprising hydrophobicallymodified polymers are largely undeveloped.

As is shown in papers [13-15] from other technical fields, hydrophobicaggregates formed in diluted solutions of hydrophobically modifiedpolymers and polymer/surfactant complexes may be destroyed inwater-organic mediums. For example, it is shown in papers [13, 14] thatin dilute solutions of hydrophobically modified polyacrylamide, thehydrophobic aggregates are destroyed when 20-50 vol. % of acetonitrileis added to the water. This solvent is used during the determination ofthe molecular weight of individual macromolecules by gel-permeationchromatography (GPC). Similar behaviour is observed inpolymer/surfactant complexes. Paper [15] discloses that in water/ethanoland water/isopropanol mixtures, containing 40-50 vol. % of alcohol,destruction of the polymer/surfactant complexes takes place because of aweakening of hydrophobic interactions.

SUMMARY OF THE INVENTION

The present invention is at least partly based on the recognition that asimilar approach can be used to control the formation of physical gelsin polymer-containing gelable liquids, with the aims of (1) slowing downthe process of gelation in an aqueous medium and (2) suppressinggelation upon contact with hydrocarbons.

In a first aspect, the present invention provides a controlled-gelationaqueous solution containing:

-   -   a hydrophobically-aggregating gelling agent, and    -   an effective amount of an inhibitor which suppresses hydrophobic        aggregation of the gelling agent, the inhibitor being soluble in        water so that when the controlled-gelation aqueous solution        contacts aqueous media the inhibitor disperses and gelation        occurs, and the inhibitor further being substantially insoluble        in hydrocarbons so that when the controlled-gelation aqueous        solution contacts hydrocarbon media the inhibitor continues to        suppress said hydrophobic aggregation and gelation is        suppressed.

A further aspect of the present invention provides for the use of thecontrolled-gelation aqueous solution of the previous aspect as ahydrocarbon well service fluid.

In a further aspect, the present invention provides a method of forminga controlled-gelation solution, the method comprising:

-   -   providing an aqueous solution containing a        hydrophobically-aggregating gelling agent, and    -   introducing into the aqueous solution an effective amount of an        inhibitor which suppresses hydrophobic aggregation of the        gelling agent, the inhibitor being soluble in water so that when        the aqueous solution contacts aqueous media the inhibitor        disperses and gelation occurs, and the inhibitor further being        substantially insoluble in hydrocarbons so that when the aqueous        solution contacts hydrocarbon media the inhibitor continues to        suppress said hydrophobic aggregation and gelation is        suppressed.

Yet another aspect of the present invention provides a method forselectively blocking water inflow from a water-producing formation of ahydrocarbon well, the method comprising:

-   -   injecting into the well a controlled-gelation aqueous solution        containing (a) a hydrophobically-aggregating gelling agent        and (b) an effective amount of an inhibitor which suppresses        hydrophobic aggregation of the gelling agent and which is        soluble in water but substantially insoluble in hydrocarbons,    -   whereby when the controlled-gelation aqueous solution contacts a        hydrocarbon-producing formation the inhibitor continues to        suppress said hydrophobic aggregation so that gelation is        suppressed, and when the controlled-gelation aqueous solution        contacts a water-producing formation the inhibitor disperses so        that gelation occurs and the formation is blocked.

Preferably, the aqueous solution of any one of the previous aspectscontains 1 to 10 wt % of the gelling agent. The gelling agent may be awater-soluble co-polymer comprising hydrophilic and hydrophobic monomerunits. The aqueous solution may further contain a surfactant which formsa complex with the water-soluble co-polymer.

The present invention provides in a further aspect a method forselectively inhibiting the gelation of an associating gelling liquidcomprising hydrophobically associating substances, which arehydrophobically modified water-soluble polymers or complexes of suchpolymers with a surfactant,

-   -   the method ensuring, in the case of contact of the gelling        liquid with hydrocarbon media, maintenance of the inhibiting        effect so gelation does not take place, while, in the case of        contact of the gelling liquid with aqueous media, disappearance        of the inhibiting effect so that gelation takes place,    -   the method comprising introducing an inhibitor into the liquid        prior to bringing it into contact with said media, the inhibitor        suppressing the hydrophobic association of hydrophobically        associating substances, and being soluble in aqueous media but        substantially insoluble in hydrocarbon media.

An aqueous associating gelling liquid capable of selective gelation isprovided in a further aspect of the invention, the liquid comprising:

-   -   1-10 wt. % of hydrophobically associating substances which are        hydrophobically modified water-soluble polymers or complexes of        such polymers and a surfactant, and    -   an effective amount of a gelation inhibitor,    -   wherein the inhibitor suppresses hydrophobic association of the        hydrophobically associating substances, and dissolves in aqueous        media but is substantially insoluble in hydrocarbon media.

Another aspect of the invention provides a composition for treatment ofan oil well, the composition including an aqueous associating gellingliquid that is capable of selective gelation, the liquid comprising:

-   -   hydrophobically associating substances which are hydrophobically        modified water-soluble polymers or complexes of the aforesaid        polymers with a surfactant, and    -   an effective amount of a gelation inhibitor,    -   wherein the inhibitor suppresses hydrophobic association of        hydrophobically associating substances, and dissolves in aqueous        media but is substantially insoluble in hydrocarbon media.

Yet another aspect of the present invention provides a method forselectively blocking water inflow into an oil producing well fromunderground water-bearing formations, the method comprising:

-   -   pumping the composition described above for treatment of an oil        well into the well bore,    -   whereby the associating gelling liquid of the composition forms        blocking gel plugs only in those places of the well where the        gelling liquid is in contact with an aqueous medium and does not        form plugs in places of contact between the associating liquid        and oil. Thus the invention may be used in the oil producing        industry to limit the inflow of water into an oil producing well        while maintaining the oil inflow substantially unchanged.

The associating gelling liquid and method for selectively inhibiting thegelation of such a liquid, make it possible to control the rate offormation of a physical gel so that the process of gelation is dependenton the type of medium (water or hydrocarbon) the gelling liquid is incontact with.

The inhibitor suppresses hydrophobic association of the hydrophobicallyassociating substances in the liquid, and has good solubility in waterbut is poorly soluble in hydrocarbons. In this way, when the associatinggelling liquid comes into contact with water, the concentration of theinhibitor in the gelling liquid falls as the inhibitor passes from thegelling liquid into the water. This causes gelation of the liquid, whichhad previously been suppressed by the inhibitor. On the other hand, whenthe gelling liquid comes into contact with hydrocarbons, gelation doesnot take place as the inhibitor is insoluble in hydrocarbons and remainsin the liquid.

In one embodiment of any one of the previous aspects, the inhibitor isan organic solvent. Organic solvents may be classified according totheir polarity, that is, their permanent dipole moment. Examples ofmoderately polar solvents include dimethylformamide, dimethylacetamide,acetonitrile, methanol, ethanol etc. Examples of relatively non-polarsolvents include dichloromethane, tetrahydrofuran, and ethyl acetate.Examples of non-polar solvents include alkanes, benzene, toluene, andcarbon tetrachloride. Preferably, the organic solvent is moderatelypolar, such that it mixes with water but is insoluble in hydrocarbons.Thus the solvent may be dimethylformamide, dimethylacetamide,dimethylsulfoxide, N-methylpyrrolidone, acetonitrile, methanol, ethanol,etc., or a mixture thereof. Preferably, the solvent is methanol, ethanolor acetonitrile.

In another embodiment of any one of the previous aspects, the gellingliquid comprises, as a hydrophobically associating substance, ahydrophobically modified water-soluble polymer based on polyacrylamide,having 84.4 mol. % of acrylamide units, 1.5 mol. % ofn-dodecylacrylamide units and 14.1 mol. % of sodium acrylate units,wherein the polymer is obtained by the method of micellar polymerizationwith a sodium dodecylsulfate concentration of 1.5 wt. % and a totalconcentration of the monomers of 3.0 wt. %. Additionally, a solution ofcetylpyridinium chloride surfactant may be added to the gelling liquidin the amount of 70 μl of the surfactant solution with a concentrationof 0.02 mol/l per 4 ml of the gelling liquid.

In another embodiment of any one of the previous aspects, the gellingliquid comprises, as a hydrophobically associating substance, ahydrophobically modified water-soluble polymer based on polyacrylamide,having 88.8 mol. % of acrylamide units, 1.5 mol. % ofn-dodecylacrylamide units and 9.7 mol. % of sodium acrylate units,wherein the polymer is obtained by the method of micellar polymerizationwith a sodium dodecylsulfate concentration of 3.0 wt. % and a totalconcentration of the monomers of 3.0 wt. %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the frequency relationship of the components of the complexmodulus of elasticity of the polymer solution of Example 1 when incontact with an aqueous medium.

FIG. 2 shows the frequency relationship of the components of the complexmodulus of elasticity of the polymer solution according to Example 1when in contact with a hydrocarbon.

DETAILED DESCRIPTION OF THE INVENTION

In order for the physical gel formed by the controlled-gelation solutionof the present invention to have sufficient mechanical properties,hydrophobically modified polymers may be used as the gelling agent. Inone embodiment these polymers are water-soluble co-polymers comprisinghydrophilic and hydrophobic monomer units.

The hydrophobically modified polymers should comprise a sufficientamount of groups with pronounced hydrophobic properties, which groupsare capable of providing reliable cross-linkage of the polymer chains byforming hydrophobic aggregates, or complexes of such hydrophobicallyassociating polymers with a surfactant. One of the possible types ofhydrophobically modified polymers is based on polyacrylamide and has thefollowing formula:

wherein R is a C₅₋₂₀ (preferably C₇₋₁₅) saturated or unsaturatedaliphatic alkyl group, X is —O— or —NH—, and Y is —H or —CH₃. TypicallyR is saturated. Typically R is linear. In one embodiment, R is a—(CH₂)_(n)CH₃ group where n is 8 or 11. x may be in the range 0.5 to 5(preferably 1 to 3), and y may be in the range 2 to 25 (preferably 5 to15). The “x” monomer unit is hydrophobic and the “y” monomer unit ishydrophilic. Q is a group which forms charged species in aqueoussolution. For example, Q may be a carboxylate, sulphate, sulphonate,monophosphate, monophosphonate, xanthate, or ammonium group. The groupmay be in the form of a salt or the corresponding acid/base. Theammonium group may be primary, secondary, tertiary or quaternaryammonium group. In one embodiment Q is a —COONa group, although the Naatom can be replaced by a hydrogen atom, a different alkali metal, suchas K, or any other suitable cation-forming species. R′ links Q to thepolymer backbone chain. R′ may be a covalent bond or a relatively smallorganic (preferably hydrocarbyl) group, e.g. one containing 1, 2, 3, 4or 5 carbon atoms. Preferably R′ is a covalent bond. However, in oneembodiment, R′ is a —NH—C(CH₃)₂—CH₂— group, with the —NH— group beingconnected to the polymer backbone chain and the —CH₂— group beingconnected to Q which is e.g. a sulphonate group.

The proposed associating gelling liquid (which may be acontrolled-gelation aqueous solution) and method for selectivelyinhibiting gelation of the gelling liquid may be used for the selectiveblocking of the inflow of water into an oil-producing well fromsubterranean water-bearing formations while maintaining the flow of oilunchanged. The inhibitor makes it possible to suppress hydrophobicassociation which is responsible for gelation. Therefore, prior to andas the liquid is pumped into the well, the viscosity of the liquid isinsignificant, which facilitates handling and placement of the liquid.Furthermore, the presence of the inhibitor in the gelling liquid canimprove the wettability of the liquid with downhole formations, whichpromotes more effective penetration of the liquid into the “workingsection” of the oil producing well.

After a period of time, the liquid forms blocking gel plugs at thoseplaces in the well where the liquid is in contact with aqueous media.However, at places of contact between the liquid and oil, gelationcontinues to be suppressed by the inhibitor and gel plugs are notformed.

The invention is further illustrated by the following non-limitativeexamples.

EXAMPLE 1

0.2 g of terpolymer I comprising 84.4 mol. % of acrylamide, 1.5 mol. %of n-dodecylacrylamide and 14.1 mol. % of sodium acrylate, synthesizedby the method of micellar polymerization with a concentration of sodiumdodecylsulfate of 1.5 wt. % and a total concentration of the monomers of3.0 wt. %, was dissolved in 10 ml of a mixture of ethanol (60 vol. %)and water (40 vol. %) with continuous stirring for 2 hours. In order tomodel the contact of the polymer solution with formation water, 1 ml ofthe ready polymer solution was added to 20 ml of an aqueous saltsolution containing 30 g/l of sodium chloride and 3 g/l of calciumchloride. In order to model the contact with oil, 1 ml of the polymersolution was added to 20 ml of n-heptane.

After two weeks the frequency relationships of the complex modulus ofelasticity of the polymer solutions in contact with the aqueous saltsolution and the n-heptane were measured. These are presented in FIG. 1and FIG. 2. For the sample in contact with water, the modulus ofelasticity in the frequency range of from 0.01 to 10 Hz was higher thanthe modulus of losses, i.e., the sample was a physical gel. In contrast,for the sample in contact with the n-heptane, the modulus of elasticityin the frequency range of from 0.01 to 10 Hz was lower than the modulusof losses, i.e., in this sample gelation did not take place. The valuesof the modulus of elasticity measured at a frequency of 0.1 Hz, and alsothe viscosity of the initial polymer solution, are presented in Table 1.

EXAMPLE 2

0.060 g of terpolymer I (composition as defined in Example 1) wasdissolved in 3.8 ml of a mixture of methanol (53 vol. %) and water (47vol. %). The change of the rheological properties of the solution uponcontact with an aqueous salt solution and n-heptane was studied in thesame way as described above in relation to Example 1. The measurementdata are presented in Table 1.

EXAMPLE 3

0.3 g of terpolymer II comprising 88.8 mol. % of acrylamide, 1.5 mol. %of n-dodecylacrylamide and 9.7 mol. % of sodium acrylate, synthesized bythe method of micellar polymerization with a concentration of sodiumdodecylsulfate of 3.0 wt. % and a total concentration of the monomers of3.0 wt. %, was dissolved in 10 ml of a mixture of ethanol (40 vol. %)and water (60 vol. %) with continuous stirring for 2 hours. The changeof the rheological properties of the solution upon contact with anaqueous salt solution and n-heptane was studied in the same way asdescribed above in relation to Example 1. The measurement data arepresented in Table 1.

EXAMPLE 4

A gelling mixture based on terpolymer I was prepared as described inExample 1, with the exception that 70 μl of a 0.020 mol/l aqueoussolution of the surfactant cetylpyridinium chloride was additionallyadded to 4 ml of the mixture. The change of the rheological propertiesof the solution upon contact with an aqueous salt solution and n-heptanewas studied in the same way as described above in relation to Example 1.The measurement data are presented in Table 1. TABLE 1 Comparison ofmoduli of elasticity of example samples after contact with an aqueousmedium and with n-heptane. Viscosity of G′ of sample at frequency of 0.1Hz, Pa initial solution, Contact with Contact with n- Example Pa · saqueous medium heptane 1 0.13 67 0.16 2 0.52 5.3 0.36 3 0.33 23 0.47 40.18 12.0 0.85Thus, similar results (the formation of a physical gel upon contact withwater and the absence of gelation contact with n-heptane) are obtainedfor different hydrophobically modified polymers, and also forhydrophobically modified polymer/surfactant complexes, in the presenceof different inhibitors.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

REFERENCES

All references mentioned herein are incorporated by reference.

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1. A method for selectively blocking water inflow from a water-producingformation of a hydrocarbon well, the method comprising: injecting intothe well a controlled-gelation aqueous solution containing (a) ahydrophobically-aggregating gelling agent and (b) an effective amount ofan inhibitor which suppresses hydrophobic aggregation of the gellingagent and which is soluble in water but substantially insoluble inhydrocarbons, whereby when the controlled-gelation aqueous solutioncontacts a hydrocarbon-producing formation the inhibitor continues tosuppress said hydrophobic aggregation so that gelation is suppressed,and when the controlled-gelation aqueous solution contacts awater-producing formation the inhibitor disperses so that gelationoccurs and the formation is blocked.
 2. A method according to claim 1,wherein the aqueous solution contains 1 to 10 wt % of the gelling agent.3. A method according to claim 1, wherein the gelling agent is awater-soluble co-polymer comprising hydrophilic and hydrophobic monomerunits.
 4. A method according to claim 3, wherein the water-solubleco-polymer has the formula:

and wherein: R is a C₅₋₂₀ saturated or unsaturated aliphatic alkylgroup, X is —O— or —NH—, Y is —H or —CH₃, Q is a group which formscharged species in aqueous solution, and R′ is a covalent bond or arelatively small organic group containing 1, 2, 3, 4 or 5 carbon atoms.5. A method according to claim 3, wherein the aqueous solution furthercontains a surfactant which forms a complex with the water-solubleco-polymer.
 6. A method according to claim 1, wherein the inhibitor isan organic solvent.